The preparation of star-branched polymers comprising a nucleus joining polymeric arms of non-hydrogenated and hydrogenated homopolymers or copolymers of conjugated dienes or selectively hydrogenated copolymers of conjugated dienes and mono-alkenyl arenes is known. The nucleus of these polymers comprises a poly (polyvinyl aromatic) coupling agent such as poly(divinylbenzene) and can have 4 to 30 polymeric arms. The polymeric arms are polybutadiene or polyisoprene arms.
In general, there are numerous publications and patents in the general area of acrylates and methacrylates prepared by anionic polymerization. U.S. Pat. No. 4,927,703, U.S. Pat. No. 4,826,941 and Fayt et al., Macromolecules Vol. 20, 1442-1444 (1987) are illustrative. These citations disclose the use of lithium chloride and crown ethers to prepare narrow polydispersity poly (meth)acrylates. For example, at page 1442 of Fayt et al., it is noted that by the use of lithium chloride one can prepare polymethacrylates with a polydispersity of 1.2-1.6 as compared to 3.61 without lithium chloride.
WO 96/23008 discloses a batch process for the anionic polymerization or copolymerization of acrylates and methacrylates. In WO 96/23008, the monomers or comonomers are added to the anionic polymerization reaction medium either at once or in a rapid continuous manner (not drop-by-drop). This reference does not teach the continuous tubular reactor process of the present invention.
EP 0 524 054 teaches a process for the polymerization of (meth) acrylic monomers consisting of polymerizing the monomers in the presence of an initiation system comprising (1) at least one initiator and (2) an alkali metal alcoholate. The reference requires the presence of a metal alcoholate and does not teach the continuous tubular reactor process of the present invention.
Multifunctional anionic initiators (Escwey et al., Makromol. Chem. 173 (1973) 235 and Burchard et al., Polymer 16 (1975) 180) have been used as well as controlled amounts of bis-unsaturated monomers, usually divinyl benzene (Worsfold et al., Can. J Chem., 47 (1969) 3379, Young et al., Macromolecules, 11 (1978) 899, Martin et al., Anionic Polymerization: ACS Symp. Ser., 166 (1981) 557). Most of the work in the area of star-branched polymers has been conducted using polystyrenes or polydienes. Relatively little work has been done with polymethacrylate star-branched polymers. Rather polydisperse materials which increased in viscosity and molecular weight on storage were prepared using group transfer polymerization (Simms et al., Chem. Technol. 64 (1991) 139). Zhu et al., Macromolecules, 25 (1992) 7330, Tsitsilianis et al., Macromolecules, 24 (1991) 5897 and Zilliox et al., J Polym. Sci., Part C, 22 (1968) 148 have published reports in which star polymers from methacrylates were prepared.
U.S. Pat. No. 5,552,491, incorporated herein by reference, teaches star-branched acrylate and methacrylate polymers and their use as lubricating oil additives. The acrylic monomers and initiator system taught in this patent are suitable for use in the present invention. However, this patent teaches a batch method for preparing the star-branched polymers. There is no teaching in this patent of the continuous polymerization process of the present invention.